The present invention relates to the capacitor arts. It finds particular application in conjunction with titanium, aluminum, tantalum and other metal sponges for capacitor anodes grown in the form of dendrites on metallic substrates, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to the growth of sponges for a variety of applications in which a high accessible surface area to volume ratio is desired.
Electrical devices, such as power supplies, switching regulators, motor control-regulators, computer electronics, audio amplifiers, surge protectors, and resistance spot welders often require substantial bursts of energy in their operation. Capacitors are energy storage devices that are commonly used to supply these energy bursts by storing energy in a circuit and delivering the energy upon timed demand. Typically, capacitors consist of two electrically conducting plates, referred to as the anode and the cathode, which are separated by a dielectric film. In order to obtain a high capacitance, a large dielectric surface area is used, across which the electrical charge is stored. The capacitance, C of a capacitor is determined by the formula: ##EQU1##
where Q is the electrical charge and V is the voltage between the plates. Capacitance is proportional to the charge-carrying area of the facing plates, A, and is inversely proportional to the gap width, X, so that ##EQU2##
where (.di-elect cons..multidot..di-elect cons..sub.0) is a proportionality constant, .di-elect cons..sub.0 is the permittivity of vacuum (value=8.85 .times.10.sup.12 Farad/m), and .di-elect cons. is the relative permittivity or dielectric constant for a dielectric substance. High capacitance capacitors should have a large area, A, and a thin dielectric film with a high dielectric constant.
Commercial capacitors attain large surface areas by one of two methods. The first method uses a large area of thin foil as the anode and cathode. See, e.g., U.S. Pat. No. 3,410,766. The foil is either rolled or stacked in layers. In the second method, a fine powder is sintered to form a single slug with many open pores, giving the structure a large surface area. See, e.g., U.S. Pat. No. 4,041,359. Both these methods require considerable processing to obtain the desired large surface area. In addition, the sintering method results in many of the pores being fully enclosed and thus inaccessible to the dielectric.
Metallic sponges provide an opportunity for increasing the surface area over conventional capacitor materials. Metallic sponges of titanium, such as those produced by the Hunter and Kroll processes, have relatively large surface areas. However, due to the random growth patterns, surface areas are not maximized and a considerable portion of the surface is inaccessible, being fully enclosed by the sponge. Additionally, chemical residues from the process generally remain on the sponge, and may be trapped within the enclosed pores or within remotely accessible pores.
To be effective as an energy storage device, a capacitor should have a high energy density (Watt-hours per mass) or high power density (Watts per mass). Conventional energy storage devices tend to have one, but not both, of these properties. For example, lithium ion batteries have energy densities as high as 100 Wh/kg, but relatively low power densities (1-100 W/kg). Examples of energy storage devices with high power density are RF ceramic capacitors. Their power densities are high, but energy densities are less than 0.001 Wh/kg. The highest energy capacitors available commercially are the electrochemical supercapacitors. Their energy and power densities are as high as 1 Wh/kg and 1,000 W/kg, respectively.
The dielectric film within the capacitor serves as the energy storage medium. Energy density is the amount of stored energy per unit volume of dielectric. To maximize the energy density of a capacitor, it is desirable to have a dielectric with a large surface per volume, a high dielectric constant, and a high dielectric strength. The energy density is a function of the dielectric constant and the dielectric strength, as follows: EQU Energy density=dielectric constant.times.(dielectric strength).sup.2 (3)
A good capacitor geometry is one in which the dielectric is readily accessed electrically, that is, it has a low equivalent series resistance that allows rapid charging and discharging. High electrical resistance of the dielectric prevents leakage current. A good dielectric, therefore, has a high electrical resistance which is uniform at all locations. Additionally, a long-term stability (many charging-discharging cycles) is desired. Conventionally, dielectrics tend to become damaged during use.
The present invention provides a new and improved capacitor having an anode formed from a directionally-grown metallic sponge which provides high surface area and much improved energy storage capacity over conventionally known capacitors and a dielectric film with good electrical properties which has the ability of self repair in the event of a breakdown in the dielectric film.